Influence of soil scour on lateral behavior of large-diameter offshore wind-turbine monopile and corresponding scour monitoring method

Soil scour is a major cause of foundation failures for offshore wind turbines (OWT), and has emerged as a significant concern for safety analysis and design. Most previous studies focus on investigating scour mechanisms but not their consequences. Therefore, this paper mainly investigated the scour effects on the lateral behavior of large-diameter offshore monopile and the corresponding scour monitoring method. Firstly, the effect of the soil scour on the lateral behavior of large-diameter monopile foundation of OWT was investigated numerically. It revealed that the horizontal load-displacement response, the bending moment and horizontal displacement of pile body, and the p-y curve were significantly affected by scour. However, the soil displacement field was not disturbed and remained "two-failure zone," i.e., wedge failure zone near the ground surface and rotational failure zone near the pile toe. Furthermore, a monitoring method for scour depth of soil seabed around monopile foundation based on natural frequency was proposed and verified through model tests. It was found that test environment and conditions had little effect on the natural frequency of the measuring rod, which only depended on its free end length. This provides a theoretical basis for in-situ monitoring of soil scour for offshore wind turbine foundation.